In a conventional personal computer, the so-called "television game" etc. are played especially among children. Such a personal computer is provided with an apparatus for displaying an image on a displaying screen which comprises a central processing unit (CPU) for supplying control signals and data which are read from a read only memory (ROM) to selected circuits included therein, a video display controller for defining patterns of a background and a sprite, color codes, displaying positions etc. on a displaying screen in accordance with signals from the CPU, a video color encoder for producing analog RGB signals and video color signals in accordance with signals from the video display controller, and a sound generator for producing sounds in accordance with sound signals supplied from the CPU.
In operation, a background and a sprites are displayed on the displaying screen which is connected to the video color encoder in accordance with a program stored in the ROM, and sounds are radiated from speakers which are connected to the sound generator in accordance with the program so that a story of a television game is developed in the personal computer.
The video display controller normally comprises a predetermined number of registers each storing a raster number at which a displaying screen is divided. On the divided regions of the displaying screen, a plurality of patterns corresponding to different pages are displayed.
In accessing a video memory, an address is increased in accordance with a counted value of an address counter by one to write data thereinto or to read data therefrom. In such a case, if the CPU is eight bits and the video memory is sixteen bits, the data is divided into upper and lower bytes which are transferred separately.
In a direct memory access (DMA), on the other hand, a predetermined block length of data are transferred from a source address of the video memory to a destination address thereof under the control of a DMA controller.
According to the conventional apparatus for displaying an image on a displaying screen, however, there are following disadvantages. A first disadvantage is that a construction of the apparatus is complicated and a cost of manufacturing the same is high for the reason why a predetermined number of registers must be provided therein to divide the displaying screen.
A second disadvantage is that an image is processed only in a single pattern thereby providing less interest in the display thereof because an address is increased by one regularly. In a case where a special processing of an image is required in a television game to give an increased interest for a player, this can not be performed in accordance with a method of a video memory access because the method of accessing the video memory is fixed as described above.
A third disadvantage is that a processing amount and time of the CPU are increased in addressing the video memory and so on because data of sixteen bits are divided into upper and lower bytes to be transferred through 8/16 bit data bus interface between the CPU of eight bits and the video memory of sixteen bits.
A fourth disadvantage is that a signal for the trigger of a DMA transfer which is advantageous in a transfer speed as compared to a CPU transfer must be set beforehand.
A fifth disadvantage is that a data bus width of a circuit included in the apparatus must be set to be a predetermined width so as to comply with that of the CPU, and a matching of an interface therebetween must be adjusted in a case where data bus widths of the CPU and the remaining circuits are different from each other.